bifcl: Fix header include guards

tools/bifcl/include/bif_arg.h

@@ -1,5 +1,4 @@
-#ifndef bif_arg_h
-#define bif_arg_h
+#pragma once
 
 #include <stdio.h>
 
@@ -47,5 +46,3 @@ protected:
 	const char* type_str;
 	const char* attr_str;
 };
-
-#endif

tools/bifcl/include/module_util.h

@@ -2,6 +2,8 @@
 // These functions are used by both Zeek and bifcl.
 //
 
+#pragma once
+
 #include <string>
 
 using namespace std;